1. Field of the Invention
The present invention relates to an optical scan apparatus and an image formation apparatus, particularly to an optical scan apparatus which scans a surface with a plurality of light beams as well as to an image formation apparatus including the optical scan apparatus.
2. Description of the Related Art
A known image formation apparatus using the Carlson process forms images by, for example, scanning the surface of a rotating photoconductive drum with a plurality of light beams to form an electrostatic latent image thereon, visualizing the electrostatic latent image as a toner image, and fusing the toner image on a sheet of paper as a recording medium. In recent years, this type of image formation apparatus has been well used in easy printing as an on-demand print system. Therefore, there has been increasing demand for formation of high-density images and speeding-up of image outputs.
In general, in order to speed up the image outputs, a print speed is increased by increasing the rotation speed of a polygon mirror deflecting the light beam and the rotation speed of the photoconductive drum. However, there is a problem that along with the increase of the rotation speed of the polygon mirror, noise and vibration from a drive system is increased as well as power consumption, resulting in lowering durability of the apparatus. Further, the speeding-up of the image outputs has a trade-off relation with formation of the high-density images so that image quality deteriorates as the rotation speed of the polygon mirror increases.
In view of the above problems, Japanese Laid-open Patent Application Publication No. 2005-250319 and No. 2004-287292, for example, have disclosed a scan method of scanning a photoconductive drum with plural light beams at once in order to concurrently realize the formation of the high-density images and the speeding-up of image outputs. The method disclosed therein employs a surface emitting laser having plural emission portions and can collectively deflect the plural light beams from the surface emitting laser to thereby scan the photoconductive drum with the plural light beams at once.
The surface emitting laser used therein such as vertical cavity surface emitting laser (VCSEL) is composed by two-dimensionally arranging the emission portions on a single element easily. Because of this, it is easier to increase the number of light beams used for a single scan thereon than an edge emitting laser; however, there is drawbacks in the surface emitting laser that it is hard to speed up the image outputs and the longevity of the element tends to be relatively short due to thermal interference between the emission portions.